1. Field of the Invention
The present invention relates to a gas compressor for air conditioning in automobiles, buildings, etc. which is adapted to compress a gas, such as a refrigerant gas, and discharge it.
2. Description of the Related Art
Examples of a gas compressor for air conditioning in automobiles, buildings, etc. include a vane rotary type gas compressor, which will be described with reference to FIG. 13. The vane rotary type gas compressor includes a gas compressing portion, which has a cylinder 5 whose inner periphery is cylindrical, a front side block 6 and a rear side block 7 which are situated at the axial ends of the cylinder 5, a rotor 11 rotatably arranged in the cylinder 5, and vanes 15 accommodated in vane grooves in the rotor 11 so as to be capable of advancing and retreating. The rotor 11, the vanes 15, and the cylinder 5 define cylinder compression chambers. The above components are contained in a front housing 1a and a rear housing 1b. The front housing 1a has a refrigerant suction port 2, and the rear housing 1b has a discharge port 3. Provided in the front housing 1a is a suction chamber 4 communicating with the suction port 2, and the suction chamber 4 communicates with the cylinder compression chambers. Further, a discharge chamber 8 communicating with the cylinder compression chambers is provided in the space defined by the rear side block 7 provided in the front portion of the rear housing 1b and the rear portion of the rear housing 1b. 
Formed in the rear side block 7 are discharge holes 7a communincating with the cylinder compression chambers, discharge passages 7b, and a release hole 7c provided to the discharge chamber 8. The gas compressed in the cylinder compression chambers is released to the discharge chamber 8 through the release hole 7c. In the discharge chamber 8, it is necessary to separate oil contained in the compressed gas released from the release hole 7c. In this regard, the gas compressor is provided with a screw type compressor using a demister 30 in order to enhance the efficiency in oil separation. The demister 30 is provided perpendicularly with respect to the direction in which the gas flows, and, while having a certain thickness, is formed as a flat plate when seen in section taken along the gas flow direction.
In the gas compressor shown in FIG. 13, when the rotor 11 is rotated, the volume of the cylinder compression chambers is varied, and a refrigerant introduced through the suction port 2 and the suction chamber 4 is compressed in the cylinder compression chambers. The compressed refrigerant is discharged from the compression chambers, and released to a release-portion-side space 8a of the discharge chamber 8 through the discharge holes 7a, the discharge passages 7b, and the release hole 7c, the oil contained in the compressed gas being separated by the demister 30. The separated oil drips and gathers in an oil sump 18, and the compressed gas from which the oil has been separated is discharged to a discharge-port-side space 8b of the discharge chamber 8, and is further carried from the discharge port 3 to an external air conditioning system through discharge piping (not shown). The refrigerant which has been discharged to the exterior circulates through the air conditioning system and returns to the suction port 2 of the gas compressor to be compressed again in the cylinder compression chambers and further discharged into the air conditioning system. By repeating this operation, air conditioning is continuously effected. Due to the difference in pressure between the suction chamber 4 and the discharge chamber 8, the oil in the oil sump 18 is sent from an oil supply hole 9 provided at the bottom of the rear side block 7 to sliding portions and gap portions of the gas compressing portion, serving to prevent wear in the compressor and functioning as a seal in the form of an oil film. In this process, part of the oil joins the compressed gas and moves with the same. [Patent Document 1 JP 11-294362 A]
As compared with the other types of compressors, e.g., the swash plate type, scroll type, and screw type ones, the vane rotary type compressor is smaller and lighter in weight. However, due to the large difference in pressure between the adjacent compression chambers, it is subject to leakage of the refrigerant gas (inner leakage) between the adjacent compression chambers. Thus, in the vane rotary type compressor, it is necessary to effect sealing by using high viscosity oil in order to prevent this inner leakage. As a result, the mixture gas consisting of the refrigerant gas and oil mist and discharged from the compression chambers exhibits high viscosity. Due to its high viscosity, the mixture gas undergoes, when it strikes the demister, an increase in frictional force when passing therethrough due to the oil adhering to the demister, with the result that the load on the demister due to the jet stream thereto increases. As a result, there may be a case where the demister fails to maintain its form and is pushed away in the gas flow direction (to the downstream side) to undergo a reduction in its density, resulting in a marked deterioration in oil separation efficiency. Further, when the oil exhibits an extremely high viscosity because of its low temperature as in the case of operation start or when a liquid refrigerant is compressed, the oil from the compression chambers strikes the demister in the liquid state. In such case, due to the large force of the jet stream, the demister undergoes not only deformation but breakage. Generally speaking, oil used in a refrigerant compressor exhibits a kinematic viscosity as follows. The viscosity of the mixture of oil and refrigerant during operation varies according to the kind of refrigerant, temperature, pressure, etc. The ratio of the viscosity of the mixture of oil and refrigerant during operation in a vane rotary type compressor to that in other types of compressor is much the same as the ratio of the kinematic viscosity of the oil alone (refrigeration oil) in a vane rotary type compressor to that in other types of compressor as shown in Table 1. The temperature of the mixture gas after compression in normal operation is approximately 70 to 110° C., whereas the temperature thereof at the operation start ranges from −35 to 40° C.
TABLE 1KinematicOil for vane rotaryOil for swash plate type,viscosity of oiltypescroll type and screw type 40° C. 60-350 mm2/s15-55 mm2/s100° C.13-25 mm2/s  5-12.5 mm2/s